1. Field of the Invention
The present invention relates to a nuclear fuel assembly for use in a boiling water nuclear reactor (referred tonn as "BWR" hereinafter) and a BWR having a core loaded with such a nuclear fuel assembly.
2. Description of the Related Art
Improvement in nuclear power station economy can effectively be achieved by a reduction in the fuel cycle cost through enhancement of the fuel exposure.
It would be possible to enhance the exposure simply by increasing enrichment of conventional fuel. The increase in the enrichment, however, causes hardening of neutron spectrum, resulting in occurrence of phenomena such as (a) increase in the absolute value of the void coefficient, (b) increase in the reactivity in the core in cold state, (c) reduction in ability to control the reaction by burnable poison such as gadolinia. Such phenomenon may undesirably lead to reduction in thermal margin and shut-down margin of the nuclear reactor. Enhancement of exposure merely by increase in the enrichment is inevitably accompanied by increase in the cost of natural uranium per fuel, as well as in the cost incurred in the course of enriching of the fuel, so that the effect in reducing the fuel cycle cost achieved through fuel enrichment is diminished.
This problem is dealt with in BWRs by softening neutron spectrum through increasing water-to-fuel ratio. More specifically, it is effective to increase the water region of water rods in a fuel assembly. Increase in the water region of water rod in the center of a fuel assembly having fuel rods arranged in 8 rows and 8 columns (referred to as "8.times.8 fuel assembly" hereinafter) can be realized only at the cost of decrease in the fuel rods, resulting in a reduced thermal margin. Furthermore, degree of freedom in the nuclear design is disadvantageously reduced due to, for example, restriction in the arrangement design of gadolinia-containing fuel, because the number of fuel rods per assembly is decreased. Fuel assemblies improved to achieve higher fuel economy have been proposed in, for example, Japanese Patent Publication No. 3-78954 and U.S. Pat. No. 5,198,186. These fuel assemblies employ water rods of different cross-sectional shapes, but has a common feature in that each fuel rod has upper and lower end regions of natural uranium, and an enriched uranium region between these natural uranium regions, the enriched uranium region having upper, middle and lower sections, the upper and lower sections having average enrichments across horizontal cross-section which are equal to each other and lower than that of the middle section, the content of burnable poison per unit axial length being lower in the upper section than in other sections of the enriched uranium region. For instance, U.S. Pat. No. 5,198,186 discloses a fuel assembly in which an area where four fuel rods can be disposed is occupied by a single water rod of an increased diameter.
The fuel assembly disclosed in U.S. Pat. No. 5,198,186 exhibits an average discharge exposure of 38 GWd/t. An attempt to further enhance the exposure through increase in the average enrichment in this fuel assembly is encountered with a problem in that the thermal margin is reduced due to increase in the area of the water region of the water rod at the center of the nuclear fuel assembly.